If the gov't can't keep drugs out of maximum security prisons, they'll never be able to keep them off the street.
That said, drugs are harmful to individuals who rely on their mind to survive... who don't have the claws and teeth of tigers or the agility of the gazelle. Taking drugs is analogous to pulling the tigers' teeth and breaking the gazelles' legs.
Fatty foods and lack of exercise are harmful to the health of productive individuals. The government has no right to tell me what to eat or when or how much to exercise. My body, my choice.